For eastbound trips, the waiting time is quoted in minutes (e.g. "The next train to North Station will arrive in 5 minutes"). On the Riverside line, the westbound trips west of Kenmore are also quoted in minutes. However, in the subway, the westbound times are quoted in stops (e.g. "The next (pause) B (pause) train is (pause) one (pause) stops away". Why? Patrons don't know how long a "stop" is. At Park Street, when the sign says, that a B train is one stop away, it's meaningless because I can see the B train waiting on track 3. I don't know whether it will start in 1 minute or 8 minutes.

Or if I'm at GovCen and a C train is three stops away, what does that mean because C trains start their journey two stops away.

It was stated that they are still trying to work out all of the issues with Green line predictions when they went live, and that the "stops away" was used due to excessively inaccurate predictions. I've noted that even where you do get time predictions (eastbound in the subway) they're less accurate than the heavy rail lines. My most recent experience was a Lechmere train beating it's prediction by 3 minutes as it arrived Boylston. Also seen this happen at North Station with Lechmere-bound service.

There are two different styles of countdown used on the Green Line. It's a fundamental consequence of the geometry and dispatching on the line, not any oversight of laziness on the part of those implementing the signs.

Minute countdowns are used on the D Branch, Kenmore, Hynes, Copley, Arlington, and eastbound from Boylston to Science Park. These work essentially the same as the countdowns on the heavy rail lines. However, they do not start counting down until a train has left Riverside eastbound or Park westbound.

The westbound sides at Boylston through Science Park have the "stops away" system. Because trains often hold in the turnbacks at North Station, Government Center, and Park, and because trains can pass each other at Park - and all of those are manual dispatch decisions - there is no way for an automated system to predict the order and timing of westbound trains until they are past Park. (That may change in a year or two with some different dispatching methods, but no guarantees.) So using stops away gives the best indication to passengers as to what to expect.

How often are the estimates updated? When a sign at Copley tells me that a Lechmere train is 3 minutes away, is that based upon its Heath Street departure, or its Symphony departure.

I also realize that Murphy's Law always applies. If two trains are headed eastbound for Copley, the system doesn't know whether the D or the E train will get there first. OTOH, I've never seen a sign east of Copley out of sequence, so I assume the system is updated when a train gets to Copley.

In a similar situation last Sunday afternoon, I was at Fenway waiting for a Riverside car. One was not due for 12 minutes and no red lights were visible when I looked towards Longwood. That means that headway was at least 14 minutes, a bit long when the Red Sox are playing. When I looked up five minutes later, the next train was due in one minute with the second train now seven minutes away. As the train was almost empty, I assumed it came from the Kenmore loop.

About 3 months ago, I was at Kenmore when the message was, "Heath Street train arriving". It was empty and didn't stop. I wonder where it went.

It is updated every time a train passes the AVI unit leaving a station in the tunnel, or the GPS point leaving a surface station, if I remember correctly. So that Copley arrival is based on Symphony, not just on Heath Street.

Occasionally a driver will have a wrong AVI code that doesn't get noticed for a while, because some switches can be manually set. It's more common with out-of-service trains, as you saw. I've seen Heath Street appear a few times on D branch surface signs.

An eastbound D train is likely to remain a D train - switching eastern terminals is common, but except for the beginning and the end of runs, westbound terminals are generally fixed. So the system knows that an eastbound train coming from Riverside, at Park eastbound, is three stops (GC EB, GC WB, Park WB) away from Park westbound. If the train is routed to loop at Park instead, the system will pick up on that from the changed AVIs, and report it as a D train one stop away.

After picking up a friend at Woodland on Friday (tracks have nice fencing around them, not hard to imagine fare gates there!) after an 80 minute ride from Government Center) I had to take the D Line yesterday from Brookline Hills to Fenway Park and back. So I timed arrival compared to the countdown clocks. Inbound was advertised as "5 minutes" away, actual was 10, and we c r a w l e d all the way to Fenway (train was packed, and this was 3 hours before first pitch). Outbound (during 6th inning) was advertised as 6 minutes away, was actually 11. Was also packed, even though game crowds were not onboard yet.

Is this a common issue? I can't say I've taken the Green line more than a 5 times since last fall.

The system doesn't know how many people are at Kenmore/Fenway and cannot tell how long an inspector or dispatcher will hold the train. They can't account for varying dwell times either. They can only guess based on typical occurrences.

You can see why the "stops away" system is used when you go to the fringes of some of the heavy rail lines - during off-peak times the inbound countdown at Porter can sometimes get stuck at 3 minutes while a train is sitting in Alewife. Therefore, Science Park could be stuck on "1 minute" even if a train will sit at Lechmere for another 10. I imagine that GLX will do away with some of this (at least at the current locations), but since to my understanding the clocks run off train position without incorporating scheduled departure time, the outermost stations on any line are likely to stay hit-or-miss for the time being.

Now, a different question would be "how useful is saying 'n stops away' compared with saying nothing," given that I would imagine most riders care less about where the train is and more about when it will arrive.

I'd rather have the "stops away" thing since as a rider it's harder to know if the info is accurate or not, but at least there's a chance! With them saying "5" minutes when it's really 10, I know the info is wrong. Lie to me all you want, just call me pretty while you do so!